Apple keeps a hidden database of old operating systems. Clicking the official link below forces the Mac App Store to open a hidden product page: Official Mac App Store macOS Catalina Link Using MDS or gibMacOS
: It was the first version to run on a dedicated read-only system volume , significantly increasing protection against malware by preventing apps from modifying core system files. How the Download Worked macos catalina 1015 19a583 mac app store dow
Then relaunch App Store.